In recent years, mask patterns have become finer and thinner along with the progress in size reduction of semiconductor devices. Thus, not only the two-dimensional measurement of a mask pattern shape such as a line width, but also the measurement of the height of the mask pattern which affects transfer characteristics and the like is becoming increasingly important.
Techniques for measuring the height of the pattern include: (1) a method of detecting the height by observing a cross section of a sample with a scanning electron microscope (SEM); (2) a method of measuring the height with an atomic force microscope (AFM); (3) a method of measuring the height by capturing a plurality of SEM images with different focus conditions and subjecting the SEM images to image processing as disclosed by Japanese Laid-open Patent Publication No. 05-299048; and the like.
However, the method (1) in which the cross section of the sample is observed with the SEM has the following problems. A long time is required to start the observation of the cross section, and the sample used in the measurement cannot be used as a product since this method is a destructive inspection.
Moreover, in the method (2) of measuring the height with the AFM, although the sample can be measured in a nondestructive manner, a long time is required for the measurement and the throughput is low. In addition, the method (2) has a problem that a probe wears down as the number of times of the measurement increases and the measurement accuracy thereby deteriorates.
Furthermore, the method (3) using the SEM images captured with different focus conditions has a problem that accurate measurement cannot be performed when the thickness of the pattern is smaller than the focal depth of the SEM.